This invention relates to improvements in industrial solid state logic systems. In many prior systems of this type, one or more signals from the output stage of a logic gate are impressed on the control element of a solid state switch such as a triac in order to control the current supplied to a load through the triac. Generally, such systems translate low level signals of the order of 1 milli-ampere at 4 to 7 volts from the logic system into high level outputs of 2 amperes at 50 to 220 volts for the purpose of energizing loads such as solenoid valves, motor starters, electromagnetic brakes and the like. It is desirable that such output switches only produce an ON state energizing the load when driven by a logic "1" signal.
Many of these logic systems operate by a current sinking technique which gives rise to problems when connected to solid state switches or output drivers. Current sinking refers to the completion of a current path to ground or common which effects a change in the voltage level at a junction in the circuitry thereby to control the output component. An output driver used with current sinking logic is switched ON if the output of the logic gate controlling the driver is at logic "1" and OFF if the output of the logic gate is at logic "0" thereby sinking current from the driver. In such current sinking systems, if the conductor connected to the input of the driver is broken, the logic gate loses control and the driver output then assumes an ON state which erroneously energizes the load.
When source current is used to energize the driver an open circuit will not cause an erroneous driver "ON" state.
Most logic systems are adapted to sink current for their basic operation and only to source enough current to prevent leakage current from degrading the logic voltage noise margins. Therefore it would be desirable to use only the source current from the logic output to produce a logic "1" signal which will turn the driver ON.
Both AC and DC drivers having built in indicator lamps pose two additional problems:
1. THE DISSIPATION IN THE LAMP, E.G. 110V at 20 mA=2.2 watts, is comparable with the total dissipation in the output component, and therefore severely limits the power handling capability of the device, since half or more of the temperature rise can be caused by the indicating lamp alone;
2. the driver can switch any voltage e.g. 6,12,24, or 48 V DC or 48/110V AC within the limits of the output component, but the indicator is effective at only one nominal voltage and must be interchangeable if the driver is to be used over a wide voltage range.
It also would be desirable to have the indicator electrically close to the output component, so that when the driver indicates an ON state, one could be reasonably sure that the output component was conducting. If the indicator were energized by the input signal to the driver, the indicator could be on while the output component was in fact off due to a driver amplifier failure, thus creating a false impression of the driver output state.
The advantages of a light-emitting diode arrangement are:
a. The dissipation in the light emitting diode is typically low, e.g. 40 milli watts, and therefore does not contribute much heat or temperature rise, nor limit the output power handling capability of the output component. b. The driver can be used over a wide voltage range without affecting the indication. c. The system can be checked out with the load power off.
With the exception of failure of the actual output component, this indication does not give rise to a false impression of the driver output state, since it is electically very close to the output component.